Glass-ceramics have been utilized for over 30 years. U.S. Pat. No. 2,920,971 (Stookey) originally disclosed the preparation of glass-ceramics through the heat treating of precursor glass bodies. As explained therein, glass-ceramic articles are prepared in three general steps: (1) a glass-forming batch, normally containing a nucleating agent, is melted; (2) that melt is shaped into an article and cooled to a temperature below the transformation range of the glass; and (3) that glass article is heat treated at temperatures above the annealing point of the glass, and often above the softening point of the glass for a sufficient time to cause the glass to crystallize. The heat treatment can be scheduled so as to control the size and, in some instances, the identity of the crystals developed. Therefore, the crystal structures present in a glass-ceramic article can be the result of both the base chemical composition of the precursor glass and the heat treatment the glass body is subject to.
Glass-ceramics have been utilized in the manufacturing of such varied articles as cookware, tableware, missile nose cones, protective shields, and industrial applications. Recently, the utilization of glass-ceramics has expanded in the computer and electronics field. Currently glass-ceramics are being investigated for use as substrate materials in magnetic memory storage devices such as computer hard drive systems. Generally, a magnetic memory storage device consists of two fundamental units: a head pad and a rigid information disk. The head pad supports an element capable of reading/writing data magnetically on the information disk, while the information disk embodies two basic components, specifically a rigid substrate and a magnetic media coating on the surface of the rigid substrate.
Today's market for rigid magnetic storage is well established with advances foreseen through the use of thin film media technology. Increased information densities, higher disk rotation speeds, and lower head flying heights afford greater data storage and retrieval efficiencies and demand extremely tight tolerances in substrate specifications for flatness, rigidity at high rotational velocities, surface texture, and stabilized thermal expansion. Where the product is designed for the high performance market, high capacity and rapid access characteristics are key requirements. Current market trends toward smaller hard drives call for thin, lightweight, rugged disks that have high functional densities and are capable of withstanding multiple takeoffs and landings without deterioration of the magnetic media coating and the memory storage.
Recent research has led to the development of glass-ceramic materials suitable for use as substrates in magnetic memory devices. For example, U.S. Pat. No. 4,971,932 (Alpha et al.) discloses a rigid information disk, consisting essentially of a rigid substrate possessing a surface coating of magnetic media. That reference particularly describes two different types of glass-ceramic material suitable for use as the substrate material, those containing crystals having a chain silicate and those containing crystals having a sheet silicate as the predominant crystal phase.
Further, applicants' U. S. Pat. No. 5,476,821 (Beall et al), describes glass-ceramics having properties well suited for use as information disk substrates. These materials provide good fracture toughness and Knoop hardness values, a Young's modulus of 14-24.times.10.sup.6 psi, and are capable of taking a fine polish. The ability to take a fine polish with a minimum amount of finishing time and effort is of very great importance in terms of the economics of producing an information disk. An information disk must have an ultra smooth surface upon which the magnetic media is coated to permit proper operation of the memory device. In order to produce an economically viable information disk, a glass-ceramic substrate material must be able to meet information disk requirements and qualifications with a minimum of finishing time and effort expended in the polishing and grinding of the information disk surface. It has been shown that the utility of many glass-ceramics as an information disk substrate is economically disadvantageous because of the increased cost in terms of man-hours, materials and efforts that are required to finish the surface.
It has also been found that glass and glass-ceramics used as information disk substrates should preferably be alkali-free. In the past it has been noted that the glass-ceramic should be free of alkali ions in that alkali ions present in a glass-ceramic substrate tend to degrade and interrupt the performance of the magnetic media coating that is placed on its surface.
As is preferred with glass-ceramics used in traditional articles it is also true with glass-ceramics used as information disk substrates that the glass-ceramic should have a stabilized thermal expansion over a wide range of temperatures. The utility and applicability of a glass-ceramic to a wide range of uses, environments, and articles of manufacture is greatly increased when the glass-ceramic has a stable thermal expansion which is exhibited by a thermal expansion curve free of points of inflection or flexion. The thermal expansion plot of a preferred glass-ceramic in which the change in dimension is plotted versus the temperature of the glass-ceramic is a straight line having no change in slope. Such a stabilized thermal expansion is very important in that glass-ceramic articles are normally exposed to a wide range of temperatures that can vary from below 0.degree. C. to above 800.degree. C. Such a stabilized thermal dimensional expansion allows for the use of such a glass-ceramic in a broader range of articles and allows for the glass-ceramic to be in contact with or bonded with other materials and substances throughout a viable temperature range. Further, a stabilized thermal expansion helps to prevent expansion cracking of the glass-ceramic. A glass-ceramic or ceramic with a non-stabilized thermal expansion is prone to complete structural failure. This prevents the use of it in applications where it would be subjected to thermal cycling, such as in refractory uses. Particularly with information disks, a stabilized thermal expansion allows for the preservation of the bond between glass-ceramic substrate surfaces and the magnetic media coating. Further, a stabilized thermal expansion allows for the proper mounting of the information disk on the center spindle which is normally made from a substance different than the information disk substrate. U.S. Pat. No. 5,028,567 (Gotoh et al.) describes the utility of a glass-ceramic which is substantially free of flexion in the thermal expansion curve.
Glass-ceramic articles containing hexacelsian have been discussed in the past as evidenced by U.S. Pat. No. 4,360,567 (Guillevic) and U.S. Pat. No. 3,272,610 (Eppler et al.). "Compositional Study and Properties Characterization of Alkaline Earth Feldspar Glasses and Glass-Ceramics" by Dov Bohat, published in Vol. 4 (1969) of the Journal of Materials Science, pp. 855-860 and "Transmission Electron Microscopy of SrAl.sub.2 Si.sub.2 O.sub.8 : Feldspar and hexacelsian polymorphs" by Jutta Topel-Schadt et al., published in Vol 13 (1978) of the Journal of Materials Science, pp 1809-1815 further disclose the hexacelsian crystal structure.
The broad application and use of hexacelsian glass-ceramics has been hindered by the instability of the thermal expansion behavior of hexacelsian. As discussed and shown in "High-Temperature Modification of Barium Feldspar", by Yoshiki and Matsumoto, published in Vol. 34, No. 9 of the Journal of the American Ceramic Society, pp. 283-286, hexacelsian (hexagonal crystal form of BaO--Al.sub.2 O.sub.3 --2SiO.sub.2) experiences a discontinuous change in its thermal expansion at 300.degree. C. As noted, such a drastic change must be considered when using hexacelsian in ceramic applications. This unstable jump or severe slope change in the thermal expansion of hexacelsian at approximately 300.degree. C. is often referred to as a hook in the expansion curve. It has presented in the past a difficulty with the usefulness of hexacelsian crystals in a glass-ceramic or ceramic material. This hook in the hexacelsian thermal expansion normally makes the utilization of hexacelsian as a primary crystal in glass-ceramic information disk substrates difficult and often disadvantageous because of warping and structural failure.
Accordingly, it is the primary objective of the present invention to disclose a glass-ceramic article having a primary crystal phase with a hexagonal sheet structure characteristic of hexacelsian and having a stabilized thermal expansion.